U.S. patent number 5,276,537 [Application Number 07/828,363] was granted by the patent office on 1994-01-04 for diamondlike carbon thin film protected hologram and method of making same.
This patent grant is currently assigned to Physical Optics Corporation. Invention is credited to David G. Pelka, Christopher C. Rich, Gajendra D. Savant.
United States Patent |
5,276,537 |
Savant , et al. |
January 4, 1994 |
Diamondlike carbon thin film protected hologram and method of
making same
Abstract
A diamondlike carbon thin film protected hologram comprises an
organic film deposited on a substrate and recorded to form a
hologram, and a diamondlike carbon thin film deposited on the
hologram, or first and second substrates having a diamondlike
carbon thin film deposited thereon and an organic film having a
holographic pattern recorded therein and sandwiched between the
diamondlike carbon thin films of the first and second substrates. A
method of fabricating a diamondlike carbon thin film protected
hologram comprises forming a layer of dichromated gelatin on a
substrate, recording the dichromated gelatin to form a hologram,
and depositing on the hologram a diamondlike carbon thin film at
room temperature, or depositing a diamondlike carbon thin film on
first and second substrates, forming a layer of dichromated gelatin
on the thin film of one of the substrates, recording the layer to
form a hologram, and adhering the thin film of the second substrate
to the hologram. The diamondlike carbon thin film protected
hologram of the present invention is impervious to moisture and
humidity and other environmental conditions which would otherwise
negatively affect the performance of the hologram.
Inventors: |
Savant; Gajendra D. (Torrance,
CA), Rich; Christopher C. (San Pedro, CA), Pelka; David
G. (Los Angeles, CA) |
Assignee: |
Physical Optics Corporation
(Torrance, CA)
|
Family
ID: |
25251590 |
Appl.
No.: |
07/828,363 |
Filed: |
January 30, 1992 |
Current U.S.
Class: |
359/3; 359/1;
359/507; 427/577; 428/408; 430/1 |
Current CPC
Class: |
G03H
1/0252 (20130101); G03H 2250/39 (20130101); G03H
2260/10 (20130101); Y10T 428/30 (20150115); G03H
2250/12 (20130101) |
Current International
Class: |
G03H
1/02 (20060101); G03H 001/02 (); G03H 001/04 ();
B32B 009/00 (); B32B 031/00 () |
Field of
Search: |
;428/408 ;427/160,577
;359/1,3,2,15,507,512,513,22,27,28,32 ;430/1,2 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Lerner; Martin
Assistant Examiner: Parsons; David R.
Attorney, Agent or Firm: Nilles & Nilles
Claims
We claim:
1. A process for fabricating a dichromated gelatin hologram
comprising the steps of:
forming a layer of dichromated gelatin on a substrate:
recording the dichromated gelatin to form a hologram;
depositing on the hologram a diamondlike carbon thin film at room
temperature.
2. The process as defined in claim 1 wherein the substrate is
flat.
3. The process as defined in claim 1 wherein the substrate is
curved.
4. A process for fabricating a hologram comprising the steps
of:
pouring a viscous solution of dichromated gelatin on a
substrate;
spinning the substrate to form a uniform film of dichromated
gelatin;
drying the film;
exposing the film to a monochromatic light source to record a
desired pattern in the film;
processing the exposed thin film by dipping in fixer solution and a
series of water, water and alcohol, and alcohol baths;
drying the processed film; and
forming a protective layer on the dried film, the protective layer
comprising a room temperature deposition of diamondlike carbon thin
film.
5. A protected hologram comprising:
a substrate;
an organic film deposited on the substrate, the film having a
holographic pattern recorded therein; and
a diamondlike carbon thin film deposited on the film.
6. The hologram as defined in claim 5 wherein the film is selected
from the group consisting of dichromated gelatin, photoresist,
silver halide, POLAROID DMP-128 recording material, or DuPont
photopolymar recording material.
7. A process for fabricating a dichromated gelatin hologram having
first and second substrates, each substrate having two sides, the
process comprising the steps of:
depositing on one side of each of the first and second substrates a
diamondlike carbon thin film;
forming a layer of dichromated gelatin on the thin film of the
first substrate;
recording the layer to form a hologram;
adhering the thin film of the second substrate to the hologram.
8. The process as defined in claim 7 further characterized by
depositing a diamondlike carbon thin film on the other side of each
of the first and second substrates.
9. The process as defined in claim 7 or 8 wherein the first and
second substrates comprise polycarbonate.
10. A process for fabricating a dichromated gelatin hologram having
first and second substrates, each substrate having two sides, the
process comprising the steps of:
depositing on one side of each of the first and second substrates a
diamondlike carbon thin film;
preparing a free-standing dichromated gelatin layer;
recording the layer to form a hologram;
sandwiching the hologram between the thin films on the one side of
each of the first and second substrates.
11. The process as defined in claim 10 further characterized by
depositing a diamondlike carbon thin film on the other side of each
of the first and second substrates.
12. The process as defined in claim 10 or 11 wherein the first and
second substrates comprise polycarbonate.
13. A protected hologram comprising:
first and second substrates;
a diamondlike carbon thin film deposited on each of the first and
second substrates;
an organic film having a holographic pattern recorded therein and
sandwiched between the diamondlike carbon thin films of the first
and second substrates.
14. The hologram as defined in claim 13 wherein the film is
selected from the group consisting of dichromated gelatin,
photoresist, silver halide, POLAROID DMP-128 recording material, or
DuPont photopolymer recording material.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention pertains to the protection of holograms made from
dichromated gelatin. More particularly, this invention pertains to
a diamondlike carbon thin film applied as a protective layer to
dichromated gelatin holograms.
1. Background of the Prior Art
It has been demonstrated that dichromated gelatin (DCG) is a unique
holographic recording material that can simultaneously demonstrate
high reflectivity (greater than 99.5%) and broad bandwidth from
near UV to near IR of the electromagnetic spectrum (0.3 .mu.m to
2.8 .mu.m). These properties are fully spelled out in the following
references incorporated herein by reference.
1. T. Jannson, I. Tengara, Y, Qiao and G. Savant, "Lippmann-Bragg
Broadband Holographic Mirrors," J. Opt. Soc. Am. A/Vol. 8, No. 1,
201-211 (1991).
2. B. J. Chang, "Dichromated Gelatin Holograms and Their
Applications", Optical Engineering, Vol. 19, No. 5, 642-648
(1980).
3. B. J. Chang and C. D. Leonard, "Dichromated Gelatin for the
Fabrication of Holographic Optical Elements", Applied Optics Vol.
18, No. 4, 2407-2417 (1979).
4. G. Savant, T. Jannson and Y. Qiao, "Super-high Resolution
Holographic Materials for UV and XUV Applications," in Practical
Holography, III, S. A. Benton, Ed. Proceedings Soc. Photo-Opt.
Instrum. Eng. 1051, 148-155 (1989).
5. J. Jannson, T. Jannson, and K. H. Uy, "Solar Control Tunable
Lippmann Holowindows," Solar Energy Materials 14, 289-297
(1986).
Among the many holographic recording materials that are currently
available, DCG is the best irreversible material due to its
excellent transparency, high diffraction efficiency, high signal to
noise ratio, cost effectiveness, and availability of dichromated
gelatin in almost grainless form, and its high spatial resolution
with a uniform MTF between 100 and 5000 lines per mm. DCGs's
dynamic range is very large, and its index modulation can reach as
high as 0.2-0.5O. Salminen, and T. Keinonen, "On Absorption
Refractive Index Modulation of Dichromated Gelatin Gratings," Opt.
Act. 29, 531-40 (1982). As a result of these unique properties, DCG
holograms can be used in myriad applications including high channel
density wavelength division multiplexing (U.S. Pat. No. 4,926,412),
diffraction coherence filters (U.S. Pat. No. 4,958,892), broad band
single mode couplers (U.S. Pat. No. 5,018,814), Lippmann
holographic mirrors (U.S. Ser. No. 456,175, issued as U.S. Pat. No.
5,083,219), optical interconnects (U.S. Pat. No. 4,838,630), and
numerous other important uses.
Such combination of properties is not displayed by currently
available polymer-based holographic recording materials such as
DuPont photopolymer, POLAROID DMP-128 recording material, Hughes'
Polymer System, polyvinyl carbosols, or polyvinyl alcohol-based
holographic recording systems. The composition of DuPont
photopolymer is known and identified as "DuPont photopolymer" by
those skilled in the art, and consists of a binder, initiator,
monomer, sensitizer, and plasticizer as fully described in Smothers
et al., "Photopolymers for Holography" and Weber et al., "Hologram
Recording in DuPont's New Photopolymer Materials," Practical
Holography IV, SPIE OE/Lase Conference Proceedings, 1212-03 and 04,
Los Angeles, Calif., Jan. 14-19, 1990. The composition DMP 128 is
also known and identified as DMP 128 by those skilled in the art,
and consists of a dye sensitizer, a branched polyethylenimine as a
polymerization initiator and a free radical polymerizable ethylenic
unsaturated monomer as described in U.S. Pat. No. 4,588,664 and
Ingwall et al., "Properties of Reflection Holograms Recorded in
Polaroid's DMP-128 Photopolymer," SPIE Vol. 747 Practical
Holography 11 (1987). Unlike synthetic polymer-based holographic
recording materials, however, DCG is not immune to humidity and
moisture in the atmosphere where DCG holograms are placed or
stored. DCG holograms, in general, are affected by moisture or
humidity of greater than 45% at room temperature. This problem is
more severe when the temperature is higher than room temperature,
say greater than 35.degree. C. If both temperature and humidity are
high, the rate of decay caused by moisture or humidity is quite
high.
When a DCG hologram comes in contact with moisture, it loses its
efficiency, i.e., its diffraction efficiency which is usually
99.5%, drops to as low as 90%, which makes it useless for certain
applications like eye protection, Raman filters, etc. The problem
lies in that the processed hologram has high and low crosslinked
alternating planes of high and low refractive index (high material
density and low material density) within the coating. When moisture
is present at the coating, the high material density areas soften
which lowers the refractive index of that area causing it to
average out with the rest of the hologram. Because refractive index
modulation decreases with high humidity and high temperature,
thereby decreasing hologram efficiency, it is critical to protect
DCG holograms from humidity and moisture.
Since the DCG holographic optical elements (HOE) fabrication
process is labor-intensive and expensive, it is but natural to find
ways to extend the life of DCG-HoEs. The useful life of HOEs has
been extended by protecting them from moisture by either laminating
or hermetically sealing them so that the DCG does not come in
contact with moisture. The prior art discloses numerous ways to
protect DCG holograms from the effects of humidity, each of which
has its shortcomings.
1. Sealing the DCG hologram hermetically in a transparent box has
been employed. This procedure is lacking because it interjects
numerous interfaces which cause reflection and scattering. The
transparent box is also bulky and inconvenient. Furthermore, it is
necessary to remove all moisture from the interior of the box with
a vacuum prior to sealing.
2. Liquid adhesive coatings have also been used to protect DCG
holograms. These coatings generally comprise solvents and polymers.
Many times, the solvents adversely affect the hologram, creating
haziness on its surface. Furthermore, heat is necessary to cure the
adhesive layer to eliminate the solvent. This additional step
introduces inefficiencies into the system.
3. Epoxies have been used to protect holograms but suffer from the
same problems as liquid adhesive coatings if they contain similar
solvents. If the epoxy does not contain a solvent, epoxies
nonetheless must be cured using either heat, UV, or room
temperature for prolonged periods. High temperatures can shift the
characteristics of the hologram, such as shifting the notch to a
different set of wavelengths in a notch filter, even during low
temperature curing. Whether high temperature-short duration or low
temperature-long duration curing is used, the process is not
efficient for commercial production.
UV curing is preferred especially if epoxies such as NORLAND No.
61, 67-69 adhesive, are used. UV, however, also has a tendency to
shift the wavelength characteristics of the hologram. Furthermore,
UV curing is likely to cause uneven curing of the hologram due to
greater absorption of the UV wave by the upper layers of the
hologram which are closer to the UV source. Additionally, small
quantities of toxic gases are released creating bubbles and path
marks in the adhesive. As a result the process is unacceptable and
is generally not preferred for mass production.
Room temperature curing eliminates some of the problems of heat or
UV curing but requires extremely lengthy curing times making this
type of cure unacceptable for cost effective commercial production
as well.
4. Finally, lamination using glass, a flexible film such as MYLAR
film, or a fluoropolymer has been employed. Typically, to laminate
any of these materials to the hologram, an adhesive, such as those
discussed above, are necessary to secure the laminate to the
hologram. The laminate retards evaporation of the solvents. Also,
during lamination, moisture can get trapped between the adhesive
and the hologram or the adhesive and the laminate. Typically, glass
laminates are undesirable because they are too heavy, too brittle,
and not impact proof. MYLAR film is typically unacceptable because
it is not 100% waterproof. Fluoropolymers are difficult to use and
suffer from a high failure rate if not processed optimally.
Each of the above-means of protecting a DCG hologram almost meets
the specific conditions required to protect DCG but fails in some
regard. State of the art teaching dictates that the primary
condition that must be met to successfully protect DCG is to use a
material that is waterproof and bonds well with DCG. Typically, the
prior art has attempted to meet this condition and employed mostly
hydrophobic adhesives or epoxies as discussed above. However,
although epoxies and adhesives bond well with the DCG, they
ultimately fail because of their weak interbond strength of the
polymer epoxy chains. Due to the presence of these problems for
many years, there has been a failure to protect DCG with a
protective layer that is impervious to moisture and humidity but
that bonds well with DCG. Consequently, high efficiency DCG
holograms that are complex and costly to make, as well as more
conventional DCG holograms, have been slowly losing their
effectiveness. A protective layer for holograms that has excellent
bonding, is scratch and impact proof, and fully waterproof would be
of great benefit.
SUMMARY OF THE INVENTION
A diamondlike carbon (DLC) thin film protected DCG hologram and a
method for making a DLC thin film protected DCG hologram is
presented. Specifically, a DLC layer is deposited on a DCG hologram
at room temperature using either a single or dual ion beam direct
deposition method. DLC thin film is a known extremely hard
material, an excellent insulator, has extreme corrosion resistance,
provides extremely high heat dissipation and high temperature
resistance, is impervious to moisture and as smooth as but more
durable than TEFLON coating, and is chemically inert. The DLC thin
film protected DCG hologram is thus impervious to moisture and
humidity and results in a DCG hologram in a class by itself. The
room temperature process by which diamondlike carbon thin film is
deposited on the DCG hologram does not adversely affect the
hologram, will not cause shifting of the hologram's wavelength
characteristics, and creates a coating which is transparent from
near-UV through IR.
A process for fabricating a DCG hologram is also disclosed which
comprises the steps of forming a layer of dichromated gelatin,
recording the dichromated gelatin to form a hologram, and
depositing on the hologram a DLC thin film by a room temperature
deposition process. Additional processes for fabricating a
protected DCG hologram comprise pouring a viscous solution of
dichromated gelatin on a substrate, spinning the substrate to form
a uniform thin film of dichromated gelatin, drying the thin film,
exposing the thin film to a monochromatic light source to record a
desired pattern in the thin film, processing the exposed thin film
by dipping in fixer solution and a series of water, water and
alcohol, and alcohol baths, drying the processed thin film, and
forming a protective layer on the dried thin film, the protective
layer comprising a room temperature deposition of DLC thin
film.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic of a diamondlike carbon thin film protected
hologram of the present invention;
FIG. 2 is a schematic of a diamondlike carbon thin film protected
hologram of the present invention employing a polycarbonate
substrate;
FIG. 3 is a schematic of a diamondlike carbon thin film protected
hologram of the present invention employing a polycarbonate
substrate;
FIG. 4 is a schematic of a diamondlike carbon thin film protected
hologram of the present invention employing a polycarbonate
substrate and a DCG hologram produced by free-standing holographic
techniques;
FIG. 5 is a schematic of a diamondlike carbon thin film protected
hologram of the present invention employing a polycarbonate
substrate and a DCG hologram produced by free-standing holographic
techniques.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The diamondlike carbon (DLC) thin film protected holograms of the
present invention may be made utilizing conventional hologram
fabrication and DLC thin film production techniques. The well known
process of making holograms need not be discussed here except to
state that the process of creating a DCG volume hologram generally
includes the steps of preparing a dichromated gelatin film, coating
a substrate with the film using either spin coating, web coating,
casting, doctor's blade, or dip coating processes, each of which
should create a uniform thin film of dichromated gelatin on the
substrate. The thin film is then dried, and exposed to the desired
light source or light sources to record the desired pattern. As is
appreciated in the art, virtually innumerable patterns or
holographic fringes may be recorded in the thin film depending upon
the end use of the hologram. After exposure, the recorded thin film
is then processed using fixer solution and a series of water, water
and alcohol, and alcohol baths. The type and extent of processing
determines the additional characteristics imparted to the hologram.
An excellent source regarding holography is Collier, "Optical
Holography", Academic Press (1971) incorporated herein by
reference. The preparation, exposure and processing of DCG films is
discussed specifically at pages 293-298 of that reference. Other
helpful references are those cited in the background section above
which likewise are incorporated herein by reference. After
processing, a DLC thin film is deposited on the hologram.
The DLC thin film may be produced using the process disclosed in
U.S. Pat. No. 4,490,229 and described in M. Mirtich, et al. "Dual
Ion Beam Processed Diamondlike Films for Industrial Application",
preprint for Technology 2000, Nov. 27-28, 1990, Washington, D.C.
(NASA) incorporated herein by reference. Other various and known
dual beam or single beam ion deposition processes for depositing
DLC films may also be used to coat the DCG hologram. A DCG
protected hologram 10 comprising a substrate 12, such as glass,
polycarbonate, or others, DCG hologram 14, and DLC coating 16 is
shown in FIG. 1.
In the ion beam process generally, a mixture of hydrocarbons and
argon is supplied to an ion source where a plasma is produced.
Electrically charged grids placed at one end of the ion source
extract the ions and accelerate them toward the substrate to be
coated. The coated surface is maintained at or near room
temperature since it is remote from the energetic plasma within the
ion source. Carbon and hydrocarbon ions combine on the substrate
surface to produce a dense, hard form of carbon, resulting in a
material with chemical and physical properties similar to diamond
but without long range crystalline order. The ion beam process can
be controlled to produce dense films with excellent substrate
adhesion and high optical transparency.
Because the DLC thin film is applied to the hologram after the
hologram is recorded and processed, the DLC thin film may be
applied to the hologram at a site distant from the site of
manufacture of the hologram. Of course, the processed hologram may
be exposed to moisture and humidity in the interim which will begin
the decay process. It would be beneficial to coat the completed
hologram with the DLC thin film as soon as is practicable after
processing the hologram.
The thickness of the DLC thin film applied to the hologram is
dependent upon the ultimate use of the hologram and the harshness
of its ultimate environment. The ion beam deposition process may
achieve film thicknesses as great as 1.5 .mu.m or more. Spectral
transmittance analysis of DLC films generated using the single and
dual beam ion source system, show that a 1500 .ANG. thick dual beam
film has greater transmittance at all wavelengths when compared to
a 1500 .ANG. thick single beam film. Some thinner DLC films (800 to
1500 .ANG. thick) look clear to yellow-like in appearance and films
thicker than 1500 .ANG. usually look brown. The transmittance of a
500 .ANG. thick film was found to be greater than 90% at
wavelengths greater than 7000 .ANG.. Of course, in the usual case a
DLC coating for a DCG hologram should have as high a transmittance
as possible, but the exact degree of transmittance may be different
for different applications. The infrared transmittance of DLC films
has been shown to be 100% transmitting in the wavelength region
between 2.5 and 20 .mu.m.
The chemical and physical properties of DLC films make them an
excellent, although unexpected, protective coating for DCG
holograms. DLC films are, by their nature, hydrophobic, and
therefore it was unexpected that there would be sufficient bonding
between the DLC film and the DCG hologram. Despite its hydrophobic
nature, the DLC film bonds strongly with the DCG hologram. Due to
its hydrophobic nature, the DLC film makes the DCG hologram
waterproof.
Particularly in the case of polycarbonate substrate-based
holograms, various combinations of DLC coated polycarbonate
substrates and methods of forming such combinations may be
employed. Polycarbonate substrates, unlike glass substrates, do not
provide the necessary protection from humidity or moisture. One
embodiment, shown in FIG. 2, comprises polycarbonate substrate 18,
DLC layer 20, DCG hologram 22, adhesive 24, polycarbonate substrate
26 and DLC layer 28. To fabricate this protected hologram,
polycarbonate substrate 18 is coated with DLC layer 20, which is
then coated with DCG layer 22. DCG layer 22 is then recorded to
form a hologram. Polycarbonate substrate 26 is then coated with DLC
layer 28 and that layer (with the substrate 26) is then adhered to
DCG layer 22 with adhesive 24 which may be NORLAND 61 adhesive or
another suitable adhesive. The DCG layer 22 is thus thoroughly
encapsulated between the polycarbonate/DLC layers 18, 20 and 26,
28.
A similar embodiment, shown in FIG. 3, comprises polycarbonate
substrates 30 and 32, DLC layers 34A, 34B and 36A, 36B, DCG layer
38, and adhesive 40. To fabricate this protected hologram,
polycarbonate substrates 30 and 32 are coated on both sides thereof
with DLC layers 34A, 34B and 36A, 36B respectively. A DCG layer 38
is then coated on DLC layer 34B and is then recorded to form a
hologram. DLC layer 36B of polycarbonate 32 is then affixed to DCG
layer 38 with adhesive 40. The DCG layer 38 is thus thoroughly
encapsulated between DLC coated polycarbonate layers 30 and 32.
Referring to FIG. 4, a DLC thin film protected hologram fabricated
using releasable hologram film techniques is disclosed. This
embodiment comprises substrates 42 and 44, DLC layers 46 and 48,
DCG layer 50, and adhesives 52 and 54. To fabricate this protected
hologram, polycarbonate substrates 42 and 44 are each coated on one
side with a DLC layer 46 and a DLC layer 48 respectively. A
free-standing DCG layer 50 is then fabricated, recorded, and
sandwiched between DLC layer 46 and DLC layer 48 on polycarbonate
substrates 42 and 44 respectively with adhesive layers 52 and 54
respectively. The free-standing recorded DCG layer 50 may be made
according to standard releasable UV cure techniques known in the
art.
A similar embodiment, shown in FIG. 5, comprises DCG layer 56 made
using releasable hologram film techniques, polycarbonate substrates
58 and 60, DLC layers 62A, 62B and 64A, 64B and adhesives 66 and
68. To fabricate this protected hologram, the free-standing
releasable UV cure hologram 56 is sandwiched between polycarbonate
substrates 58 and 60 which are coated on both sides with DLC layers
62A, 62B and 64A and 64B, respectively and using adhesive 66
between DLC layer 62B and DCG hologram 56 and an adhesive 68
between DLC layer 64B and DCG hologram 56. Each of the above
embodiments provides an encapsulated DCG hologram impervious to
moisture and humidity.
DLC films can be deposited on virtually any type of hologram in any
shape or form including DCG, DuPont photopolymer, POLAROID DMP-128
recording material, silver halide, or photoresist. DLC films cannot
only be used to coat flat substrates but can be used to coat curved
holograms, HOEs, laser protection eyeware (goggles and visors),
periscopes and innumerable other hologram forms.
The resistance of laser protection eyeware to scratches and damage
is greatly enhanced by DLC film. Current laser eye protection
strategies employed by the U.S. Army for its wide range of ocular
protection devices use multiple holographic or dielectric coatings
to protect against known threats and yet yield high photopic and
scotopic efficiency. As the number of threatening wavelengths
increases, the corresponding cost of laser protection eyeware
increases. Greater attention must in turn be focused on how to
prolong the useful lifetime of laser eyeware devices. In a typical
operating environment, the laser protection eyeware worn by Army
foot soldiers is exposed to moisture, oil, fuels, and other
materials that damage the eyeware. The DLC protected DCG hologram
of the present invention is so highly chemically resistant and
scratch resistant that laser protection eyeware coated with DLC
films may be virtually undamageable in such harsh environments.
It is to be understood that embodiments of the present invention
not disclosed herein are intended to be within the scope of the
claims.
* * * * *